6.2.1 Nerve impulses

Question 1

Describe the structure of a myelinated motor neurone

Answer 1

Question 2

Describe resting potential

Answer 2

Inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside)

Question 3

Explain how a resting potential is established across the axon membrane in
a neurone

Answer 3

● Na+/K+ pump actively transports:
○ (3) Na+ out of axon AND (2) K+ into axon

● Creating an electrochemical gradient:
○ Higher K+ conc. inside AND higher Na+ conc. outside

● Differential membrane permeability:
○ More permeable to K+ → move out by facilitated diffusion
○ Less permeable to Na+(closed channels)

Question 4

Explain how changes in membrane permeability lead to depolarisation and
the generation of an action potential - 1. Stimulus

Answer 4

● Na+ channels open; membrane permeability to Na+ increases

● Na+ diffuse into axon down electrochemical gradient (causing depolarisation)

Question 5

Explain how changes in membrane permeability lead to depolarisation and
the generation of an action potential - 2. Depolarisation

Answer 5

● If threshold potential reached, an action potential is generated

● As more voltage-gated Na+ channels open (positive feedback effect)

● So more Na+ diffuse in rapidly

Question 6

Explain how changes in membrane permeability lead to depolarisation and
the generation of an action potential - 3. Repolarisation

Answer 6

● Voltage-gated Na+ channels close

● Voltage-gated K+ channels open; K+ diffuse out of axon

Question 7

Explain how changes in membrane permeability lead to depolarisation and
the generation of an action potential - 4. Hyperpolarisation

Answer 7

● K+ channels slow to close so there’s a slight overshoot – too many K+ diffuse out

Question 8

Explain how changes in membrane permeability lead to depolarisation and
the generation of an action potential - 5. Resting potential

Answer 8

● Restored by Na+/K+ pump

Question 9

Draw / label a graph showing an action potential

Answer 9

Question 10

Describe the all-or-nothing principle

Answer 10

● For an action potential to be produced, depolarisation must exceed threshold potential

● Action potentials produced are always same magnitude / size / peak at same potential
○ Bigger stimuli instead increase frequency of action potentials

Question 11

Explain how the passage of an action potential along non-myelinated results in nerve impulses

Answer 11

● Action potential passes as a wave of depolarisation

● Influx of Na+ in one region increases permeability of adjoining region to Na+ by
causing voltage-gated Na+ channels to open so adjoining region depolarises

Question 12

Explain how the passage of an action potential along myelinated axons results in nerve impulses

Answer 12

● Myelination provides electrical insulation

● Depolarisation of axon at nodes of Ranvier only

● Resulting in saltatory conduction (local currents circuits)

● So there is no need for depolarisation along whole length of axon

Question 13

Suggest how damage to the myelin sheath can lead to slow responses and /
or jerky movement

Answer 13

● Less / no saltatory conduction; depolarisation occurs along whole length of axon
○ So nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction

● Ions / depolarisation may pass / leak to other neurones
○ Causing wrong muscle fibres to contract

Question 14

Describe the nature of the refractory period

Answer 14

● Time taken to restore axon to resting potential when no further action potential can be generated

● As Na+ channels are closed / inactive / will not open

Question 15

Explain the importance of the refractory period

Answer 15

● Ensures discrete impulses are produced (action potentials don’t overlap)

● Limits frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus)
○ Higher intensity stimulus causes higher frequency of action potentials
○ But only up to certain intensity

● Also ensures action potentials travel in one direction – can’t be propagated in a refractory region

Question 16

What are the factors that affect speed of conductance

Answer 16

Myelination

Axon diameter

Temperature

Question 17

Describe the factors that affect speed of conductance

Answer 17

Myelination
● Depolarisation at Nodes of Ranvier only → saltatory conduction
● Impulse doesn’t travel / depolarise whole length of axon

Axon diameter
● Bigger diameter means less resistance to flow of ions in cytoplasm

Temperature
● Increases rate of diffusion of Na+ and K+ as more kinetic energy
● But proteins / enzymes could denature at a certain temperature